latch S1 in position “R” or “G”. Similarly, the second
approximation is made by connecting S2 to the reference and
S3 to GND, and latching S2 according to the output of the
comparator. After three successive approximation steps have
been made the voltage level at the comparator will be within
1/2LSB of GND, and a digital word which represents the
analog input can be determined from the positions of S1, S2
and S3.
THEORY OF OPERATION
In the ADS774, the advantages of advanced CMOS technol-
ogy—high logic density, stable capacitors, precision analog
switches—and Burr-Brown’s state of the art laser trimming
techniques are combined to produce a fast, low power
analog-to-digital converter with internal sample/hold.
The charge-redistribution successive-approximation circuitry
converts analog input voltages into digital words.
A simple example of a charge-redistribution A/D converter
with only 3 bits is shown in Figure 1.
OPERATION
BASIC OPERATION
Figure 2 shows the minimum connections required to oper-
ate the ADS774 in a basic ±10V range in the Control Mode
(discussed in detail in a later section.) The falling edge of a
Convert Command (a pulse taking pin 5 LOW for a mini-
mum of 25ns) both switches the ADS774 input to the hold
state and initiates the conversion. Pin 28 (STATUS) will
output a HIGH during the conversion, and falls only after the
conversion is completed and the data has been latched on the
data output pins (pins 16 to 27.) Thus, the falling edge of
STATUS on pin 28 can be used to read the data from the
conversion. Also, during conversion, the STATUS signal
puts the data output pins in a High-Z state and inhibits the
input lines. This means that pulses on pin 5 are ignored, so
that new conversions cannot be initiated during the conver-
sion, either as a result of spurious signals or to short-cycle
the ADS774.
Analog
Input
Comparator
SC
L
o
g
i
4C
2C
C
Out
Signal
S
c
S1
G
S2
G
S3
G
R
R
R
+
–
Reference
Input
FIGURE 1. 3-Bit Charge Redistribution A/D.
INPUT SCALING
Precision laser-trimmed scaling resistors at the input divide
standard input ranges (0V to +10V, 0V to +20V, ±5V or
±10V) into levels compatible with the CMOS characteristics
of the internal capacitor array.
The ADS774 will begin acquiring a new sample as soon as
the conversion is completed, even before the STATUS
output falls, and will track the input signal until the next
conversion is started. The ADS774 is designed to complete
a conversion and accurately acquire a new signal in 8.5µs
max over the full operating temperature range, so that
conversions can take place at a full 117kHz.
SAMPLING
While sampling, the capacitor array switch for the MSB
capacitor (S1) is in position “S”, so that the charge on the
MSB capacitor is proportional to the voltage level of the
analog input signal. The remaining array switches (S2 and
S3) are set to position “G”. Switch SC is closed, setting the
comparator input offset to zero.
CONTROLLING THE ADS774
The Burr-Brown ADS774 can be easily interfaced to most
microprocessor systems and other digital systems. The
microprocessor may take full control of each conversion, or
the converter may operate in a stand-alone mode, controlled
only by the R/C input. Full control consists of selecting an
8- or 12-bit conversion cycle, initiating the conversion, and
reading the output data when ready—choosing either 12 bits
all at once, or the 8 MSB bits followed by the 4 LSB bits in
a left-justified format. The five control inputs (12/8, CS, A0,
R/C, and CE) are all TTL/CMOS-compatible. The functions
of the control inputs are described in Table II. The control
function truth table is shown in Table III.
CONVERSION
When a conversion command is received, switch S1 is
opened to trap a charge on the MSB capacitor proportional
to the analog input level at the time of the sampling com-
mand, and switch SC is opened to float the comparator input.
The charge trapped in the capacitor array can now be moved
between the three capacitors in the array by connecting
switches S1, S2, and S3 to positions “R” (to connect to the
reference) or “G” (to connect to GND), thus changing the
voltage generated at the comparator input.
STAND-ALONE OPERATION
For stand-alone operation, control of the converter is accom-
plished by a single control line connected to R/C. In this
mode CS and A0 are connected to digital common and CE
and 12/8 are connected to +5V. The output data are
During the first approximation, the MSB capacitor is con-
nected through switch S1 to the reference, while switches S2
and S3 are connected to GND. Depending on whether the
comparator output is HIGH or LOW, the logic will then
®
ADS774
6
BB [ BURR-BROWN CORPORATION ]
